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CBSE Questions for Class 7 Physics Heat Quiz 8 - MCQExams.com
CBSE
Class 7 Physics
Heat
Quiz 8
Electric heaters used for cooking have the filament or the heating coil on a plate made up of clay because:
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Clay is a bad conductor of heat
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Clay is a bad conductor of electricity
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Clay is necessary for good taste
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Both $$A$$ and $$B$$
Explanation
The materials which do not allow heat and electricity to pass through them easily are called insulators. For example, clay.
Electric heaters used for cooking have the filament or the heating coil on a plate made up of clay because clay is a bad conductor of heat.
State whether given statement is True or False
Two thin blankets are more warmer than one thick blanket
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True
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False
Explanation
The blanket do not insulate; actually air insulates.
Air is trapped in the space between two thin blankets. Since air is insulator of heat, it prevents the escape of body heat. The air can't pass deeper and the layers will stop the air in two thin blankets where as there are no layers and air can pass deeper in one thick blanket. That's why two thin blankets are usually warmer than one thick blanket.
State whether given statement is True or False
Radiant heat can easily pass through air
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True
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False
Explanation
It is true that radiant energy pass through air easily.
As radiant energy is type of heat radiation, it does not require any material medium in its surrounding to transfer heat from hot object to cold object. It radiates its heat, so it's easy to pass through air because radiation do not require any medium to travel.
Heat energy produces a sensation of-
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warmth
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coldness
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both warmth and coldness
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none of these
Explanation
Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases. Heat energy can be transferred from one object to another. The transfer or flow due to the difference in temperature between the two objects is called heat.
Heat energy produces a sensation of warmth
The range of a labaratory thermometer is generally from
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$${ 10 }^{ \circ} C$$ to $${ 115 }^{ \circ }C$$
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$${ -10 }^{ \circ} C$$ to $${ 110 }^{ \circ }C$$
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$${ 10 }^{ \circ} C$$ to $${ 120 }^{ \circ }C$$
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$${ -10 }^{ \circ} C$$ to $${ 150 }^{ \circ }C$$
Explanation
A laboratory thermometer, which is colloquially known as the lab thermometer, is used for measuring temperatures other than the human body temperature. It ranges from $$-10˚C$$ to $$110˚C$$. Laboratory thermometers are designed for lab purposes such as checking boiling point, freezing point, or temperature of other substances.
State whether given statement is True or False
A wooden chair feels colder than an iron chair in winter
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True
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False
Explanation
Metal conducts heat much better than wood does. Since your hand is hotter than room temperature, both the metal and the wood conduct heat away from it because iron and wood are at lower temperature as compared to your hand.So both feel cold but iron is a better conductor than wood that's why an iron chair in winter feels colder in winter than wood.
A clinical thermometer reads temperature from
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$${ 35 }^{ \circ }{C }$$ to $${ 42 }^{ \circ}{C }$$
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$${ 30 }^{ \circ}{ C }$$ to $${ 42 }^{ \circ}{ C }$$
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$${ 25 }^{ \circ}{ C }$$ to $${ 42 }^{ \circ}{ C }$$
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$${ 20 }^{ \circ}{ C }$$ to $${ 42 }^{ \circ}{ C }$$
State whether the given statement is True or False
$$\text{The upper standard point of a thermometer is 273 K}$$.
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True
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False
Explanation
The lower fixed point, or ice point, is the temperature of pure melting ice at normal atmospheric pressure.
The upper fixed point, or steam point, is the temperature of pure boiling water at normal atmospheric pressure.
$$273\ K = 0^oC$$, which is the melting point of ice.
thus, the given statement is false.
The range of clinical thermometer is $$\displaystyle 35^{0}C$$ to $$\displaystyle 42^{0}C$$
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True
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False
Explanation
The above statement is true.
The typical range of a sub-lingual thermometer for use in humans is from about $$35 °C$$ to $$42 °C$$ or $$90 °F$$ to $$110 °F.$$
Identify the statement in which the mode of heat transfer is convection ?
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The heat of the sun warming our planet
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The heat from an electric stove warming a frying pan
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Ice cubes cooling a drink
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A microwave oven cooking a meal
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An overhead fan cooling a room
Explanation
When an ice cube melts in a drink, the molecules of the drink at a higher temperature come to the ice cube and cool down. Whereas the melted particles of ice go to fulfill the empty space from where molecules of the drink have come. As the particles are moving in this heat transfer, so it is convection.
Heat can be transferred to the inside surface of the walls of the container by which of the above?
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I only
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I and II only
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I and III only
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II only
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I, II, and III only
Explanation
As the candle is burning, it radiates heat in the form of infrared radiation, and also heats the air around it, creating convection currents that reach the inner walls of the container.
Conduction of heat is not responsible for heating up the inner wall as there is no direct contact between the flame and the inner wall.
Heat travels through vacuum by:
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convection
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radiation
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conduction
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of all these
Explanation
The process of convection and conduction requires some medium made of material particle for transmission of heat. In vacuum there is no material. So, heat travels in vacuum by radiation.
State whether the following statement is True or False.
The unit of heat and temperature are the same.
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True
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False
Explanation
Unit of temperature kelvin (k) or Celsius ( ℃) or Fahrenheit (℉).
Unit of heat is calorie or Joules (J).
Hence, OPTION : B (false).
$$500 g$$ of hot water at $${ 60 }^{ \circ }C$$ is kept in the open till its temperature falls to $${ 40 }^{ \circ }C$$. Calculate the heat energy lost to the surroundings by the water. ( specific heat of water = 4200 J $${ { kg }^{ -1 } }^{ \circ }{ C }^{ -1 }$$)
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$$2400 J$$
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$$5000 J$$
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$$40000 J$$
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$$42000 J$$
Explanation
Given , $$m=500g=0.5kg , \theta_{1}=60^{0}C , \theta_{2}=40^{0}C$$
We have , specific heat of water $$c=4200J/kg-^{o}C$$
Now , heat lost to the surroundings by the water is given by the definition of specific heat c ,
$$Q=mc\Delta \theta=mc(\theta_{1}-\theta_{2})$$
or $$Q=0.5\times4200\times(60-40)=42000J$$
Before using the clinical thermometer mercury level should be below:
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$$ 45 ^\circ C$$
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$$ 35 ^\circ C$$
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$$ 95 ^\circ C$$
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$$ 55 ^\circ C$$
Explanation
The mercury level should be below $$35^{o}C$$ or $$94^{o}F$$, because minimum temperature of human body can not be below $$35^{o}C$$. Hence, it must be at lowest level to measure the temperature of human body correctly.
In case of conductors and insulators, if an electric field is applied then conduction current density$$\vec{(J)}$$ is:
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in the direction of electron flow.
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in the direction of proton flow.
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in the direction of electric field.
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against the direction of electric field.
Explanation
Current density, $$\overrightarrow { J } =\cfrac { I }{ \overrightarrow { A } } $$
Current flows in direction of flow of protons. And the direction of electric field is in the direction of flow of electrons.
And the direction of current density is in the direction of proton flow, so from above statements, current density is also in the direction of electric field.
On a cold day when a room temperature is $$15^oC$$, the metallic cap of a pen becomes much colder than its plastic body, though both are at the same temperature of $$15^oC$$, because
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metals have higher thermal capacity than plastics
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plastics have a lower density than metals
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metals are good conductor of heat
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plastics have a higher thermal conductivity than metals
Explanation
On a cold day when a room temperature is $$15^oC$$, the metallic cap of a pen becomes much colder than its plastic body, though both are at the same temperature of $$15^oC$$ because metals are good conductor of heat, so they could absorb and release heat in a better way than insulators.
The degree of hotness or coldness of a body is ......... .
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Temprature
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Coldness
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Thermometer
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Heat
Explanation
The degree of hotness or coldness of a body is Temperature.
Example
Boiling water seems much hotter than ice because boiling water temperature is at $$100^{\circ}C$$ and ice temperature is at
$$0^{\circ}C$$.
Among the following what should not be done while or before using a clinical thermometer?
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Ensure that before use the mercury level is below $$35\ ^0C$$.
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Wash the thermometer with hot water.
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Handle the thermometer with care, as you handle any glass articles.
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Don't hold the thermometer by the bulb while reading it.
Explanation
(A) The mercury level should be below $$35^{o}C$$ or $$94^{o}F$$ because the minimum reading of a clinical thermometer is $$35^{o}C$$ hence it must be at the lowest level to measure the right temperature.
(B) If we wash the thermometer with hot water, the mercury will expand due to heat given by hot water and will rise to a higher temperature. If the water is very hot, it may result in damage to the thermometer. Hence it must not be done.
(C) It must be handled with care as being made of glass and mercury inside it is poisonous.
(D) If we hold the bulb by our hand, the thermometer will take the heat from the hand and will show a wrong reading hence we should not hold it by the bulb
,
Lake water never completely freezes because
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Principle of conduction prevents it
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Principle of convection prevents it
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both of the above
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none of the above
Explanation
Lake water never completely freezes because principle of convection prevents it.
Warm water gets more dense when it gets colder and therefore sinks. This fact may lead to fact that ice should form on bottom of lake first. But water colder than $$4^oC$$ begins expanding and become less dense as it gets closer. As a result, close to freezing, colder water floats on top and warmer water sinks to bottom. Eventually, the coldest water, which has floated to top of lake in wintery condition freezes to form ice layer. When the water freezes to ice, the ice becomes significantly less dense than water can continues to float on lake's surface. And then lake never completely freezes. So, in this manner; principle of convection prevents it.
In experiment of heat transfer by convection, water inside the pipe completely gets heated (with the application of heat source). This can be explained best by which of the following statements,
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Differences in densities produced between the bottom and the top of water , in turn produce the driving force which continuously circulates the water.
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A continuous conductive channel is produced between the different layers of water in the pipe which circulates the heat.
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both the above explains correctly
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all of the above
Explanation
When the water in below part of pipe is heated, its density decreases. Hence less density water moves up (shown in red) and cooler water with more density comes down (shown in blue). It is nothing but convection as convection is mode of heat transfer in which heat is transferred in fluid (liquid or gas) by molecular motion.
Boiling water can be measured by
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Clinical thermometer
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Laboratory thermometer
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Both may be used conveniently
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Both of them can not be used
Explanation
Laboratory thermometers are used to measure temperatures or temperature changes with a high degree of precision. That's why these are mainly used in laboratories. For exp : To measure boiling points of solutions, water etc. They are made up of glass , the strength of which, is increased through thermal tempering or annealing. Types : liquid-in-glass, bimetallic strip, infrared based thermometer.
Hence, the answer is Laboratory thermometer.
A laboratory thermometer is dipped in a beaker containing hot water. As the thermometer is removed the mercury level will
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rise
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fall
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it may rise or fall depending upon how much water is kept inside the beaker
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neither rise nor fall
Explanation
As the temperature of hot water is much higher than the room temperature the level of mercury rises but when it is removed from the beaker, the mercury level starts descending as the temperature of the room is less than the temperature of hot water.
What is the measurement of temperature in Celsius scale in the above figure?
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$$25^oC$$
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$$30^oC$$
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$$26^oC$$
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$$29^oC$$
Explanation
The main scale reading of thermometer reads $$30^oC$$ and sub scale reading is zero.
the sub scale reading has 5 division on it and between them it has $$10^o$$
Hence each division measures about $$\dfrac{10^o}{5}=2^o$$
Therefore reading of thermometer is $$30^oC+0\times2^o=30^oC$$
Human body temperature is well measured by
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A clinical thermometer
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A laboratory thermometer
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Thermostat
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Both A & B can be used
Explanation
A clinical thermometer is a thermometer used to measure human body temperature. Most old ones use mercury thermometer. It is accurate and sensitive, because it has a narrow place where the mercury level rises very fast.
These thermometers are used in clinics by doctors, so they are also called a doctor's thermometers or medical thermometers. Most of them show both Celsius scale and Fahrenheit temperature scales. The scale runs from 35 degree Celsius to 42 degree Celsius.
Medical thermometers are cleaned before and after each use, with alcohol.
It has a kink which does not allow the mercury level to fall on its own.
Choose the correct statement when we measure the temperature of a water kept in a beaker:
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Thermometer should be kept vertical so that the bulb does not touch sides or bottom of the beaker
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Thermometer should be kept vertical so that the bulb touches the bottom of the beaker
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Thermometer should not be kept tilted so that the bulb touches sides or bottom of the beaker
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Thermometer should be kept tilted without touching bottom or sides of the beaker
Explanation
While measuring temperature, the laboratory thermometer should be held vertically. It should be surrounded from all sides by the substance(water in this case) whose temperature is to be measured and hence, it should not touch the sides or the bottom of the beaker.
Hence, option A is correct.
The temperature as measured by the thermometer shown here will be:
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$$84^oC$$
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$$86^oC$$
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$$88^oC$$
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$$90^oC$$
Explanation
The main scale reading of the thermometer reads $$80^oC$$
The subscale has 5 divisions in between a range of $$10^oC$$
Hence each division measures about $$\dfrac{10^oC}{5}=2^oC$$
Therefore reading of the thermometer is $$80^oC+2\times 2^oC=84^oC$$
The clinical thermometer is designed to measure the temperature of
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Human body
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Bodies kept in refrigerators
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Room temperature
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Boiling water
Explanation
A Clinical thermometer is a thermometer used to measure human body temperature. Most made in $$20th$$ century are mercury-in-glass thermometers. They are very fast with small variations in temperature. Most of these thermometers show Celsius and Fahrenheit ( medical thermometer ) scales and runs from $$35°C$$ to $$42°C.$$
Beaker A contains hot water. Beaker B contains cold water. Beaker C contains a mixture of hot and cold water. If initially your hand is put into a beaker containing ice, then putting your hand in which of the beakers A, B or C will give a sensation of warmth?
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A
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B
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C
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All of the above.
Explanation
We know that ice is solid form of water which is at 0$$^{\circ}$$C . And all other forms given in question are liquid forms and all liquid forms are above melting point of water that is
0$$^{\circ}$$C. So compared to ice all beakers are comparatively at higher temperature and will give sensation of warmth to our hand when we put our hand in any of these beakers after putting it in ice because they are at higher temperature than ice.
What is the use of kink near the bulb in clinical thermometer?
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It prevents the level of the mercury from falling when taken out of the mouth
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It allows the level of the mercury to fall when taken out of the mouth
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It allows the level of the mercury to rise when taken out of the mouth
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It prevents the level of the mercury from rising when taken out of the mouth
Explanation
Because mercury is used in clinical thermometers, and the level of mercury rises rapidly with small changes in temperature, thermometers are designed with a kink near the bulb to prevent the level of mercury from falling and ensure that the correct reading is obtained even after the thermometer is taken out of the mouth.
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